This invention relates to coating processes and apparatus, and in particular to processes and apparatus for coating surfaces of strip articles of indeterminate length.
In coating operations as herein contemplated, an initially wet, flowable coating material is applied to a surface of a substrate that is at least substantially impervious thereto, for covering the substrate surface with a continuous adherent coating layer. One especially important application of the invention, to which detailed reference will be made for purposes of illustration, is the coating of metal strip with a protective and/or decorative layer of paint or the like, prior to cutting or forming of the strip into shingles, siding or soffit panels, building trim members, or other products.
Metal strip (i.e. strips of sheet metal of indeterminate length, usually stored as coils) is continuously coated, in commercial practice, by advancing the strip longitudinally past a locality where a wet coating material such as paint is applied to one or both major surfaces of the strip, and then through a zone where the coating is cured or dried with heat. Known techniques for applying wet coating material to a strip surface include spraying, transfer from rolls, and deposit of the coating material on the strip surface immediately ahead of a doctor blade or dam which has the purpose of establishing a desired coating thickness. In the latter instance, the blade or dam, as will be understood, has a thin edge extending transversely across and very slightly spaced from the surface to be coated; the deposited wet coating material puddles on the upstream side of the blade or dam and is carried thereunder in a thin layer on the moving surface.
While coating operations using a blade or dam are advantageous from the standpoint of mechanical simplicity, they (like other coating techniques, e.g. spray and roll-coating) do not afford assured or easily attainable high uniformity of coating thickness, especially in the coating of metal strip which commonly has wavy edges, an "oil-canned" central area, or other slight deformations tending to cause variation in the effective spacing between the blade or dam edge and the strip surface and consequently in the thickness of the coating layer determined by that spacing. In order to achieve an adequate coating thickness at all points on the strip surface, therefore, it is commonly necessary to apply a coating layer having a greater average thickness (and thus to consume more coating material) than would be required if the thickness could be made more uniform. This consumption of excess coating material is economically undesirable.
An additional disadvantage of such conventional coating arrangements is the waste of coating material that occurs, e.g. through overflow, owing inter alia to shortcomings in the effectiveness of the metering action provided by these arrangements. Moreover, there is a tendency for air to be picked up in the coating material ahead of the doctor blade, and to become entrapped in the coating, especially at fast coating speeds.
Further complications are encountered when it is attempted by conventional means to provide a coating layer having a striped, streaked, marbleized or otherwise variegated pattern. It has been proposed (in U.S. Pat. No. 3,106,480) to supply paint of different colors to different locations along a common reservoir defined in the nip between two rolls, one of which transfers the paint from the reservoir to a sheet surface to be coated; but in use of blade or dam-type coating arrangements (which, as noted, offer the important advantage of mechanical simplicity) it has heretofore been considered necessary to provide separators for isolating the different colors in the coating material pool or puddle upstream of the dam, as shown for example in U.S. Pat. Nos. 2,695,005 and 3,886,898. Such separators add to the structural complexity of the coating apparatus and prevent or at least greatly restrict the provision of controlled variation in the color patterns produced.
The copending United States patent application of Carl A. Wollam and J. Lynn Gailey entitled "Coating Process and Apparatus," Ser. No. 226,699, filed concurrently herewith, and assigned to the same assignee as the present application, describes strip-coating processes and apparatus of the general type employing a dam extending transversely across a major surface of a longitudinally advancing strip, with deposit of wet coating material on that strip surface immediately ahead of the dam, wherein immediately beyond the dam, the strip is advanced longitudinally past a smooth and rigid wall (facing the coated strip surface) of extended length in the direction of strip advance, and of width at least equal to the width of the coated strip surface, and wherein during such advance past the wall the strip is uniformly restrained against movement of its coated surface more than a predetermined distance away from the aforementioned wall (i.e. in a direction normal to the direction of strip advance), such predetermined distance being equal to the desired coating thickness. The means for thus restraining the strip may, for example, comprise a second wall spaced uniformly from the first-mentioned wall so as to define therewith a gap (through which the strip advances) equal in width to the sum of the strip thickness and the desired coating thickness.
This feature of advancing the strip past a wall of extended length, immediately beyond the dam, while uniformly restraining the strip against movement away from the wall beyond a predetermined distance, is found to produce an advantageously high uniformity of coating thickness even on strip which may be wavy-edged, oil-canned, or otherwise deformed. As at present believed, the applied wet coating material, lying under pressure between one surface of the strip and the facing wall (relative to which the strip is moving), forces the strip away from the wall by a hydroplaning action, thereby (i.e. since the strip is uniformly restrained against such movement beyond a predetermined distance) smoothing out the strip deformations for the duration of advance of the strip past the wall so as to achieve substantial uniformity of spacing between all points on the coated strip surface and the facing wall. The latter spacing determines the wet thickness of the coating; hence the coating on the strip is of desirably uniform thickness, notwithstanding that the strip deformations reappear as the strip emerges beyond the wall. Also, the described process provides better metering of the coating material than conventional techniques using rolls or doctor blades; substantially all the supplied coating material is usefully consumed to provide the desired coating, with virtually no loss due to spillage over the sides. A further advantage of the described process resides in avoidance of entrapment of air in the coating.
Preferably in at least many instances, and as a further feature of the apparatus, the downstream end of the wall facing the coated strip surface (i.e. the end remote from the dam in the direction of strip advance) is a sharp edge providing an abrupt surface discontinuity rather than a radiused edge which could cause cavitation problems and resultant irregularities in the produced coating. Thus, the downstream wall edge may be constituted as the intersection of the strip-facing wall surface with a surface (of the wall structure) facing downstream and lying in a plane oriented at an angle of at least about 90.degree. to the direction of strip advance. On the other hand, the dam at or constituting the upstream end of the wall may have a rediused or chamfered edge for leading the coating material onto the strip surface and gradually initiating the fluid pressure which, between the wall and the strip surface, provides the above-described hydroplaning effect.
It is additionally found that the process and apparatus described above enable stripes and other variegated patterns of colors or shades to be achieved in the produced coating by supplying coating material of different colors or shades to different portions (spaced across the width of the strip) of a single continuous pool or puddle of the coating material extending along the inlet side of the dam, i.e. without employing any separators to isolate these different shades or colors in the pool. The nature of the patterns produced is dependent on the locations and relative quantities of the different shades or colors thus supplied, and can be controllably varied as desired during the coating of a single continuous strip surface by varying one or more of these factors.